twisted.internet.reactormodule documentation

The reactor is the Twisted event loop within Twisted, the loop which
drives applications using Twisted. The reactor provides APIs for
networking, threading, dispatching events, and more.

The default reactor depends on the platform and will be installed if
this module is imported without another reactor being explicitly installed
beforehand. Regardless of which reactor is installed, importing this module
is the correct way to get a reference to it.

New application code should prefer to pass and accept the reactor as a
parameter where it is needed, rather than relying on being able to import
this module to get a reference. This simplifies unit testing and may make
it easier to one day support multiple reactors (as a performance
enhancement), though this is not currently possible.